An energy responsive image conversion and amplification device

ABSTRACT

A solid-state electroluminescent image display device which produces the output thereof in response to an image of the light, X-ray or other types of energy projected on to said device, said output being able to be controlled by varying a DC bias voltage for said device, and which comprises an electroluminescent layer excited by an AC electric field and controllable by a DC field and an energy-responsive layer whose resistivity varies in response to the incident energy such as a photoconductive layer, said two layers being interposed between a first light-pervious electrode and a second electrode in such a manner that the electroduminescent layer is adjacent to said first electrode, a third or composite electrode place between said two layers, the AC exciting voltage being connected between said first and third electrodes and the DC bias voltage between said second and third electrodes.

United States @atent Kohashi 1 July 4, 1972 54 AN ENERGY RESPONSIVEIMAGE 3,293,441 12/1966 Kazan et al. ..250/213 CONVERSION ANDAMPLIFICATION 3,315,080 4/1967 1401135111 ..313/10s x DEVICE c/Rcu/r/000 ELECTRIC C/RCU/T Primary ExaminerRoy Lake Assistant Examiner-PalmerC. Demeo Attorney-Stevens, Davis, Miller & Mosher 57 ABSTRACT Asolid-state electroluminescent image display device which produces theoutput thereof in response to an image of the light, X-ray or othertypes of energy projected on to said device, said output being able tobe controlled by varying a DC bias voltage for said device, and whichcomprises an electroluminescent layer excited by an AC electric fieldand controllable by a DC field and an energy-responsive layer whoseresistivity varies in response to the incident energy such as aphotoconductive layer, said two layers being interposed between a firstlight-pervious electrode and a second electrode in such a manner thatthe electroduminescent layer is adjacent to said first electrode, athird or composite electrode place between said two layers, the ACexciting voltage being connected between said first and third electrodesand the DC bias voltage between said second and third electrodes.

9 Claims, 2 Drawing Figures AN ENERGY RESPONSIV E IMAGE CONVERSION ANDAMPLIFICATION DEVICE This invention relates to an energy-responsiveimage conversion and amplification device in which an electroluminescentelement is energized to luminescence with an alternating electric fieldand at the same time, the luminescent output of said element beingaffected by a DC voltage whose magnitude depends on variation in theimpedance of an energy-responsive element in relation to incidentenergy, thus the electroluminescent output from said element beingcontrolled with relation to the incident energy.

More particularly, this invention relates to an image plate forvisualizing and amplifying an image of such energy as light, x-rays orgamma rays or further for storing and displaying such an image, on theground of the above-described principle which is described in detail inthe U.S. Pat. No. 3,525,014 granted to the present applicant on Aug. 18,1970.

In conventional energy-responsive luminescent devices, control of theluminescence has been achieved, in principle, by controlling thealternating current supplied to the electroluminescent element, saidalternating current in turn being controlled according to the ACcomponent of a variation in the impedance of a photoconductive element.In such a device, however, the sensitivity of the photoconductiveelement under AC energization has been too low to be sufficientlyuseful. According to this invention, a DC control voltage issuperimposed on the AC voltage in an ingenious manner so as to permit anexpedient DC control. Thus, a remarkably improved energy-responsiveluminescent device which has satisfactory characteristics of imageconversion and amplification and which also allows for an image to bememorized therein, is obtained.

This invention will be explained hereunder in connection with anembodiment thereof and referring to the attached drawings in which;

FIG. 1 is a schematically shown sectional view of the embodiment of thisinvention shown with associated electric connection; and

FIG. 2 is a simplified electric circuit diagram equivalent to thearrangement shown in FIG. I.

Now, referring to FIG. I, a layer of AC-DC EL element 100 (this termwill be explained in the next paragraph) is provided on one surface of afirst light-pervious electrode 110, and a layer of energy-responsiveelement, for example, consisting of a photoconductive element 200, whoseresistivity varies in connection with the excitation by an incidentenergy, is positioned in the opposite side of said AC-DC EL element 100in relation to said first electrode 110 in contact with a secondelectrode 210 which is pervious to the incident energy. A divided orforaminate composite electrode 310 consisting of electroconductivemembers 301 coated with dielectric material 302 of a high resistivity isinterposed between said first and second electrodes.

In this specification, the term AC-DC EL element defines anelectroluminescent (hereafter referred to as EL) element whichcomprises, for example, EL material dispersed in dielectric medium ofresistive type or of accumulatively polarizable type which supports theinternal electric field when an external polarizing unidirectionalvoltage is applied thereto and maintains the residual component of saidelectric field when said external voltage has been removed therefrom,the waveform of the luminescent output from said element due to theexcitation with an AC electric field being controllable by a DC electricfield applied thereto.

More tangible features of the construction and the manufacturing methodof this device will be explained hereunder. Reference numeral 120indicates a support plate of transparent and heat-resistive materialsuch as glass, which is coated with said first electrode 110 whichconsists of a heatresistive and light pervious metal oxide such as tinoxide. To this first electrode 110 applied and fused is a layer of theAC-DC EL material in thickness of the order of to 50 microns, said AC-DCEL material comprising powdered dielectric medium of electro-resistiveor accumulatively polarizable type such as glass enamel containingelectro-resistive (i.e., slightly conductive) metal oxide such as SnOand further containing Li or Li and Ti, and mixed with powdered ELfluorescent material such as ZnS. Then, an electro-resistive (i.e.,slightly conductive) layer of an intermediate element 600 is appliedonto said AC-DC EL layer. Said element 600 is made from a mixture of,for example, epoxy resin and powdered electro-resistive material such aspulverized carbon or SnO or a mixture of powdered frit, an inorganicblack pigment and powder of electro-resistive material such as SnO saidmixture being bonded by heating to become an electroresistive andnon-pervious layer. Addition of ferro-electric material such as BaTiO tosaid mixture will be effective for reducing the AC impedance of theresultant layer. Said intermediate layer 600 prevents the luminescentoutput of the EL element from being fed back to the photoconductiveelement. However, in case some degree of feedback is desirable, theintermediate layer 600 is made semi-pervious accordingly. Further saidlayer 600 can be formed as a composite layer, the

additional elements being interposed between the non-pervious layer andthe AC-DC EL element 100. For example, an additional layer is made froma mixture of powder of frit or epoxy resin and powder of aferro-electric and light-reflective material (most preferably BaTiO oran electro-resistive metal oxide such as SnO said mixture being fusedand bonded by heating to become an electro-resistive reflecting layer.Thickness of the intermediate layer 600, single or composite, should be5 to 10 microns for a non-pervious layer and 10 to 20 microns for areflective layer so that the resistivity of the layer 600 across thethickness is appropriately low as compared with that of the EL element100. The intermediate element 600 may be endowed with an extremelynon-linear resistivity in order to prevent the increase of DC current.This may be achieved by the use of a non-linear resistive material suchas CdSzCl or SiC instead of the resistive metal oxide in theabove-described composition. At least one of either the resistivereflecting layer or the non-pervious layer can be eliminated in thiscase.

On the intermediate element 600, or the AC-DC EL element 100 if theintermediate element 600 is omitted, is disposed a divided or foraminatecomposite electrode 310 which consists of conductors 301, for example,of tungsten or copper wire of about 10 to 30 microns in diameter coatedwith a highly dielectric material 302 such as polyester resin or glassin thickness of, for example, 2 to 10 microns. In the presentembodiment, the composite electrode 310 is formed in the shape of a gridwith juxtaposed lines. A composite electrode 310 of another type may beformed either by reticulating the above-described coated conductors orby coating a metallic network with the above-mentioned dielectricmaterial. The space factor of the conductor 301 in said compositeelectrode 310 should be such that said electrode 310, in spite of itsforaminate formation, produces a substantially similar effect withrespect to the AC field as that with a solid plate electrode when an ACoperating voltage V is applied between the first electrode and thiscomposite electrode 310. In other words, the composite electrode 310should be constituted in such a form that the exciting AC power for theAC-DC EL element 100 is not greatly affected by the variation in theresistivity of the photoconductive element 200. The space between twoadjacent lines in a grid type composite electrode 310 as shown in FIG. 1should be preferably less than 400 microns. In a net (lattice) typeelectrode, it is desirable that the network is finer than 50 mesh. Suchspaces in the composite electrodes provide passages for the DC currentbetween the AC-DC EL element 100 and the photoconductive element 200.

In order to ensure the similarity of the composite electrode 310 to asolid plate electrode in the effectiveness for an AC field, an auxiliaryresistive layer 700 is provided in the same plane as the compositeelectrode 310 is disposed, filling at least a part of said spaces in thecomposite electrode 310. The resistivity of said resistive layer 700 ischosen at an appropriate value so that excessive dispersion of the DCcurrent can be prevented. An experiment showed that an auxiliaryresistive layer 700 having a surface resistivity of 10 to 10 ohmscm gavea satisfactory result. Accordingly, it will be understood that if thesurface resistivity of said AC-DC EL element 100 and said intermediateelement 600 fulfils the abovementioned condition, the auxiliaryresistive layer 700 is not required. Thus, the composite electrode 310can be disposed on the intermediate element 600 or the EL element 100without the auxiliary resistive layer; or further, said electrode 310can be partly or entirely sunk in said element 600 or 100. In the lattercase, the auxiliary resistive layer 700 can be assumed to be included inat least either one of the non-pervious layer or the reflective layer ofsaid intermediate element 600 or in the AC-DC EL element.

If the intermediate layer 600 and the dielectric coating 302 are made ofa vitreous material, the auxiliary layer 700 is made from powder ofepoxy resin or frit mixed with a resistive metal oxide such as SnO saidmixture being fused and bonded by heating. While, if the intermediatelayer 600 and the dielectric coating 302 are made of a binder of lowmelting point such as epoxy resin, a similar binder mixed with theresistive metal oxide is used. In this case, powder of a highlydielectric material, especially a ferro-electric material such as BaTiOmay be added to the mixture to reduce'the AC impedance of the resultantlayer. Thickness of the auxiliary layer 700 is set, for example, atabout 10 to 50 microns to limit the resistivity across the thickness.Further, the auxiliary layer 700 may be endowed with a significantlynon-linear resistivity in order to minimize the DC voltage loss acrossthe thickness and the dispersion of the DC current in the direction ofthe plane. This feature of the layer 700 may be attained by the use of anon-linear resistive material such as CdSzCl or SiC in lieu of theresistive metal oxide in the above-mentioned composition. Moreover, theauxiliary layer 700 may be endowed with functions of a non-perviouslayer and a light-reflective layer by the use of the above-mentionedmaterials selected for the respective purposes. With the use of thistype of auxiliary layer, the constitution of the device of thisinvention can be simplified.

Next, the energy-responsive element 200, that is a photoconductive layerin this embodiment is formed from a mixture of a binder such as epoxyresin and a photoconductive material such as CdS, CdSe or CdS'Se, saidmixture being applied in a layer and bonded by heating. In case thedielectric coating 302, the auxiliary resistive layer 700 and theintermediate element 600 are made of a heat-resistive material such asvitreous material as described previously, the mixture applied on thelamination formed on the support plate 120 may be heated at 600 C. forabout minutes, for example, in vacuum or in inactive atmosphere ofnitrogen, thereby to become a layer of sintered photoconductive element200 containing CdS, CdSe or CdS'Se.

The sintered photoconductive element, having substantially linearcharacteristics of voltage versus photoelectric current, makes possiblean operation with considerably high sensitivity in comparison with anunsintered element.

In a lamination in which the auxiliary layer 700 is omitted, thephotoconductive element 200 is formed in a layer filling the vacantspaces in the composite electrode 310. If the composite electrode 310 iscompletely sunken in the EL element 100 or the intermediate element 600,the auxiliary layer 700 is formed over said element 100 or 600. While,if said electrode 310 is not entirely sunken in said element 100 or 600,said layer 700 is formed filling the vacant spaces in said electrode310.

The photoconductive element 200 decreases its resistivity in response toan incident energy such as the light or X-ray. Dark-resistivity of saidelement 200 should be higher than ohms-cm, for example. Thickness ofsaid element 200 should be chosen to be 50 to 500 microns so that thedark-resistivity in the direction of thickness of said element 200 issimilar to or rather higher than the resistivity of the AC-DC EL element100 in the direction of thickness.

Then, the second electrode 210 is applied over the photoconductiveelement 200. The second electrode 210 is formed so as to be pervious ofthe incident energy including the light, X-ray and other radiative rays,and further to be electroconductive. For example, the second electrode210 is made of foil of gold or aluminum, or vapor-deposited film of ametal, or silver paint. Further, the second electrode 210 may be made inthe form of a grid comprising metallic wires of about 10 to 50 micronsin diameter juxtaposed with a space of about to 500 (If the compositeelectrode 310 has the form of a similar grid, the grid of the secondelectrode 210 should be positioned in such mannerthat the members ofsaid grid cross those of said electrode 310, or come between the lattermembers in the horizontal projection.) or in the form of a metallicnetwork. The second electrode 210 of such a foraminate formation can bepartly or completely sunken in the photoconductive element 200. Further,a second foraminate electrode has an advantage in that the variation ofthe resistivity in the direction of the plane can be utilized.

The first electrode is connected to one terminal 401 of an AC voltagesource 400, and the conductor 301 of the composite electrode 310 to theother terminal 402 of said source 400 through a feeder bar 303, thus theAC operating voltage V being applied between said electrode 110 and 310.

The second electrode 210 is connected to one terminal 501 of a DC biasvoltage source 500, while the other terminal 502 of said voltage source500 is connected to said terminal 402 of said AC voltage source 400thereby to supply a bias voltage V,, to the second electrode 210. The DCvoltage source 500 is arranged so that the DC bias voltage V is variableand the polarity of said voltage V,, can be changed to give the AC-DC ELelement 100 different operating characteristics. Thus, depending onwhether the switch 5 is in contact with the terminal p or q (refer toFIG. 1), the electrode in the luminescent output side of the AC-DC ELelement 100, that is, the first electrode 110 is biased negatively orpositively. In the former case, the device is operative mainly forconversion and amplification of an image of the energy as well aserasing of the undermentioned stored image; and in the latter case,mainly for writing of an image and the luminescent display of the storedimage.

FIG. 2 is a simplified equivalent circuit of the arrangement describedabove in connection with FIG. 1. For simplicity of the explanation,functions of the auxiliary resistive layer 700 and the intermediateelement 600 are omitted in FIG. 2. Marking R indicates resistance of thephotoconductive element 200 across its thickness, said resistance beingvariable; C capacitance across the dielectric coating 302 of thecomposite electrode 310, the conductor 301 being one electrode; and Cand R respectively capacitance and resistance across the thickness ofthe AC-DC- EL element 100.

As is seen from FIG. 2, the resistance R,. decreases in accordance withthe intensity of the incident energy L, such as the light or X-ray, andconsequently the DC bias voltage V distributed across the AC-DC ELelement is increased. As a result of this increase in the bias voltage,the waveform of the AC luminescent output L produced by AC powersupplied through the capacitance C,, is controlled so that amplitude ofsaid waveform is significantly reduced in particular half cycles of eachone cycle of the AC power.

Therefore, with this embodiment, an image L of the energy such as thelight or X-ray incident to the photoconductive element is converted andamplified with a high sensitivity and a high amplifing factor to animage of the luminescent output of negative polarity. It should be notedthat in the device of this invention, the DC bias voltage V is appliedbetween the energy-responsive element represented by the variableresistance R and the capacitive element represented by the capacitance CAccordingly, dielectric strength of the capacitance C,, on which arelatively low DC voltage corresponding to the DC bias voltage V isimposed, is not required to be very high. Further, adjustment of thesource voltage V does not affect the AC power fed to the AC-DC ELelement 100.

The above-described luminescent device having the form of a solid imageplate can be used as a pre-amplifier for a television camera equippedwith a vidicon or one of other image pickup tubes. The high sensitivityand the high energy amplifying factor of the luminescent device of thisinvention allow the conversion and amplification of an image ofconsiderably low level or the pickup of an image means for sorting,i.e., selecting and separating the luminescent pulses by means of alightchopper 1000 and a synchronous motor 1010 for driving said chopper1000 are provided. In this means, the opening interval of saidlight-chopper 1000 should be shorter than at most one cycle period ofthe operating AC voltage V, and preferably a half cycle period orshorter, said interval being adjustable. Frequency of the sortingoperation is set at'the same number as that of the operating AC voltageV,,.

As shown in the lower part of FIG. 1, the luminescent output image ofcontrolled waveform radiated from the EL element 100 in response to theincident image L is sorted in regard to particular luminescent pulsestored therein. Further, according to this invention, polarity of theobtained video signal is reversed to display a visible image of thepositive polarity relating to the incident image of the energy, on thescreen of the television system 300. Such a device utilizing the closedtelevision system is very useful as an X-ray television system for theindustrial use or the medical use.

If means for adjusting or controlling the image reproductioncharacteristics such as contrast is provided in the television system,the under-described means for sorting the luminescent pulses is notnecessarily needed.

In order to improve the reproduced image, by means of the light-chopper1000 provided in the optical system of the television camera 2000, thephotoelectric screen of an image tube in the television camera 2000 isexcited by the output image of the thus sorted luminescent pulse L In asystem wherein a television camera is used, the ratio of the framefrequency of the image tube against the sorting frequency for theluminescent pulses or the frequency of the operating AC voltage V,,should be a round number to avoid the beat or the flicker in thetelevision picture, usually the latter frequency being set at a highernumber than the former. In order to maintain such a synchronizedrelation between these two frequencies, the sweep signal orsynchronizing signal of the horizontal or vertical scanning of thetelevision system is used as the input signal E of the AC voltage source400 and the timing or synchronizing signal for the driving signal E ofthe synchronous motor 1010. That is, said signals E S and E,, areproduced from said timing signal in the respective electric circuits 410and 1020 so that the ratio of the frequency of said signals E and Eagainst that of said timing signal becomes a round number and furtherthe phasic relation between the formers and the latter is adjustable.With this arrangement, adjustability of the operating characteristicscan be attained.

In the aforegoing embodiment of this invention, the composite electrode310 is positioned between the AC-DC EL element 100 and theenergy-responsive element or the photoconductive element 200. In thisarrangement, the total thickness of the layers including theintermediate element 600, the auxiliary resistive element 700 and anyother additional elements, if desired, may be made the same as thicknessof the composite electrode or lesser than that so that the whole of saidelements can be put in the vacant space of the composite electrode 310.Such a constitution is advantageous in the point that dispersion of theDC current is prevented.

The composite electrode 310 can be partly embedded into at least eitherone of the DC-AC EL element 100 or the energy-responsive element 200.

Further, relative thickness of the composite electrode 310 is notlimited by thickness of the EL element and the photoconductive element200, but only have to be lesser than the distance between the firstelectrode and the second electrode 210. As to the position of thecomposite electrode 310, it is only re uirecl to be between the firstelectrode 110 and the second e ectrode 210 without any other limitation.

Therefore, at least either one of the EL element 100 or thephotoconductive element 200 can be contained within the vacant space ofthe composite electrode 310.

In the above-described embodiment of this invention, a photoconductiveelement is used as the energy-responsive element. However, in a deviceaccording to the principle of this invention, the energy-responsiveelement is only required to vary its resistance in response to anexcitation by any type of incident energy. Therefore, anenergy-responsive element other than the photoconductive element such asa stress sensitive resistance element or a magneto-resistance elementcan be utilized, the resistivity thereof being controlled by elasticenergy or magnetic energy respectively.

What we claim is:

1. An energy-responsive image conversion and amplification devicecomprising a first electrode which is planar and light-pervious; anelectroluminescent layer provided on said first electrode, which can beexcited by an AC electric field and waveform of whose luminescent outputis changeable by a unidirectional electric field thereacross; a secondelectrode provided on the opposite side of said electroluminescent layerin regard to said first electrode; an energy responsive layer whichvaries the resistivity thereof in response to an energy applied theretoand which is provided between said second electrode and saidelectroluminescent layer; a gridor netshaped third electrode consistingof conductors coated with dielectric material, said third electrodebeing positioned between said electroluminescent layer and said energyresponsive layer; means for applying an AC voltage between said firstand third electrodes to cause said electroluminescent layer toluminesce; and means for applying a DC voltage between said secondelectrode and said first electrode to change the waveform of the ACexcited luminescent output according to the variation in the resistanceof said energy responsive layer.

2. A device as defined in claim 1, wherein a resistive intermediatelayer is provided between said electroluminescent layer and saidenergy-responsive layer.

3. A device as defined in claim 2, wherein said intermediate layercontains ferro-electric material.

4, A device as defined in claim 2, wherein said third electrode isdisposed adjacent the interface between said intermediate layer and saidenergy responsive layer.

5. A device as defined in claim 2, wherein said third electrode isdisposed across the interface between said energy responsive layer andsaid intermediate layer.

6. A device as defined in claim 2, wherein said third electrode isembedded within said intermediate layer.

7. A device as defined in claim 1, wherein the thicknesswise resistanceof said electroluminescent layer is lower than the maximum value of thethickness-wise resistance of said energy responsive layer.

8. A device as defined in claim 2, wherein the maximum value of thethickness-wise resistance of said energy responsive layer is higher thanthe total value of the thickness-wise resistances of the remaininglayers.

9. A device as defined in claim 1, wherein said means for applying a DCvoltage includes means for changing the polarity and magnitude of thevoltage.

1. An energy-responsive image conversion and amplification devicecomprising a first electrode which is planar and lightpervious; anelectroluminescent layer provided on said first electrode, which can beexcited by an AC electric field and waveform of whose luminescent outputis changeable by a unidirectional electric field thereacross; a secondelectrode provided on the opposite side of said electroluminescent layerin regard to said first electrode; an energy responsive layer whichvaries the resistivity thereof in response to an energy applied theretoand which is provided between said second electrode and saidelectroluminescent layer; a grid- or net-shaped third electrodeconsisting of conductors coated with dielectric material, said thirdelectrode being positioned between said electroluminescent layer andsaid energy responsive layer; means for applying an AC voltage betweensaid first and third electrodes to cause said electroluminescent layerto luminesce; and means for applying a DC voltage between said secondelectrode and said first electrode to change the waveform of the ACexcited luminescent output according to the variation in the resistanceof said energy responsive layer.
 2. A device as defined in claim 1,wherein a resistive intermediate layer is provided between saidelectroluminescent layer and said energy-responsive layer.
 3. A deviceas defined in claim 2, wherein said intermediate layer containsferro-electric material.
 4. A device as defined in claim 2, wherein saidthird electrode is disposed adjacent the interface between saidintermediate layer and said energy responsive layer.
 5. A device asdefined in claim 2, wherein said third electrode is disposed across theinterface between said energy responsive layer and said intermediatelayer.
 6. A device as defined in claim 2, wherein said third electrodeis embedded within said intermediate layer.
 7. A device as defined inclaim 1, wherein the thickness-wise resistance of saidelectroluminescent layer is lower than the maximum value of thethickness-wise resistance of said energy responsive layer.
 8. A deviceas defined in claim 2, wherein the maximum value of the thickness-wiseresistance of said energy responsive layer is higher than the totalvalue of the thickness-wise resistances of the remaining layers.
 9. Adevice as defined in claim 1, wherein said means for applying a DCvoltage includes means for changing the polarity and magnitude of thevoltage.